This invention relates to needle assemblies.
The invention is more particularly concerned with spinal-epidural needle assemblies.
Spinal-epidural needle assemblies are used for the administration of spinal and epidural anaesthetic fluid. The epidural needle is inserted, in the usual way, into the epidural space. A spinal needle is then inserted through the epidural needle until its tip punctures the dura and enters the subarachnoid space. A syringe is attached to the machine end of the spinal needle and spinal anaesthetic is administered through the spinal needle to produce a rapid but short-lived anaesthetic block. The spinal needle is then removed, leaving the epidural needle in position. An epidural catheter is inserted through the epidural needle so that its patient end is located in the epidural space. The needle is then removed, leaving the catheter in position so that anaesthetic fluid can be administered, as necessary, into the epidural space to produce a longer-lasting anaesthetic effect.
Once the tip of the spinal needle has been correctly inserted, its position must be maintained while the anaesthetic fluid is administered. This can be difficult to do because, although the epidural needle is retained relatively securely by the frictional contact with the tissue through which it is inserted, the spinal needle has a relatively low-friction engagement with the bore of the epidural needle. The anaesthetist must hold the spinal needle during connection of the spinal needle to the syringe and during the administration of the spinal anaesthetic. If the needle should slip, there is the risk of it puncturing the distal wall of the subarachnoid space or of it being pulled out into the epidural space.
Various proposals have been suggested for arrangements by which the spinal needle can be fixed relative to the epidural needle once the spinal needle has been correctly inserted. In Anaesthesia 1990: 45: 593-594 and Acta Anaesthesiologica Scand 1994: 38: 439-441, J Simsa describes a metal screw clamp attached to the epidural needle hub by which the spinal needle can be fixed in position. In EP 0564859 A there is described an assembly where the spinal needle makes a bayonet-like connection with the epidural hub so that the two can be locked together, but this only enables the spinal and epidural needles to be fixed at one location. EP 0696437 describes an assembly in which the spinal needle hub has a resilient locking tab that engages between a series of teeth formed on an extension of the epidural hub. This arrangement requires the user to hold the tab in the released position while inserting the spinal needle; also, the space between the teeth limits the precision with which the spinal needle can be locked in position.